There are two kinds of optical recording system. According to one system, portions having changed physical configurations such as holes and pits are formed in an optical recording film by irradiation of an energy beam so that the arrangement of said portions may correspond to information to be recorded. Whereas according to another system, portions having changed optical characteristics such as changed refractive index or reflectance are formed in an optical recording film by irradiation of an energy beam, the arrangement of said portions corresponding to information to be recorded.
Known in the art are recording films primarily composed of a low melting point metal such as tellurium (Te) for use in optical recording media of the both systems (Japanese Patent Laid-open Publications Nos. 58-71195 and 58-9234). Te films, typical low melting point metal films, are capable of forming therein desired portions of changed physical configurations or portions of changed optical characteristics (such portions will be generally referred to herein as pits), and are very promising high sensitive materials. By the term "recording sensitivity" used herein, we mean an energy (nJ/.mu.m.sup.2) required for forming pits per unit area.
When Te is allowed to stand in ambient atmosphere, however, Te is oxidized by oxygen or moisture, and becomes transparent due to increase in transmission. When such Te is used as an optical recording film, the thickness of the film is as thin as several hudreds of .ANG.. Accordingly, if the transmission of the film is increased due to oxidation of Te, the recording sensitivity of the film is remarkably reduced. More particularly, when a thin Te film is oxidized, the melting and evaporation temperatures of the material are increased on the one hand, and due to the fact that the film has become transparent the absorption of energy such as light by the film is decreased, leading to an increased energy level required for formation of pits on the other hand. As a result the recording sensitivity of the film is remarkably reduced. For example, when a Te film is allowed to stand in an atmosphere maintained at a temperature of 70.degree. C. and a relative humidity of 85%, the recording sensitivity of the film is reduced by about 20% in about 5 hours and by about 50% in about 15 hours.
To solve the problems discussed above, various attempts have been made for the purpose of inhibiting oxidation of a Te film. There is known a procedure in which a Te film is coated with a stable inorganic material.
While this procedure is effective for inhibiting oxidation of the Te film, it is not commercially practiced since it does lower the recording sensitivity of the film and is expensive. Also known is a procedure in which a Te film is coated with a plastic material. The latter mentioned procedure is advantageous in that it does not badly affect the recording sensitivity of the film because of a low heat conductivity of the plastic material. However, oxidation of the Te film is not sufficiently inhibited because the plastic material is relatively readily permeated by oxygen and moisture.
Further, while the recording sensitivity of a recording film comprising a low melting point metal such as Te can be said high, it is not sufficiently high, and thus, optical recording films having a further improved recording sensitivity are desired in the art.